The Little Mermaid is one of my favorite Disney characters of all time because of her red hair, her curiosity and the way she does things herself even though she is told she should not/can't and she was only 16 when all of the events that changed her life forever.
The original idea was that all the mermaids were allowed to go up and see the above world once they turned fifteen. The king has six daughter's which is very similar to The Little Mermaid. Then when Ariel goes to the surface of her 15th birthday she sees a handsome prince celebrating his birthday. Also, once again very similar to Disney's version. Then a storm comes and she saves the prince, but she hides and sees that he was found by a group of girls and goes home. In the Disney movie there was no group of girls, but it was still close enough.
The youngest mermaid after saving his life is all mopping and sad because she will never get to see the handsome prince again. Her sister then discovers which kingdom the prince is from and now the mermaid can watch her prince from the sea. The possible reason Disney did not put this in is that they did not want to see girls develop obsessions over boys they can not have.
The grandma of the mermaid tells her that humans have short lifespans and can die easily and when humans die their souls go to another place, that place being possibly heaven. Whereas mer-people live for a long time and then when they die they become sea foam, which does not sound fun. This part was not in the Disney version because Disney does not like to talk about dying (other than the movies where the mothers die).
The grandma also tells her that in order to be immortal like her man is for him to fall in love with her and marry her. This is very similar to Disney. Ursula did tell Ariel that he had to kiss her from true love. So the youngest mermaid swims away from home to find the sea witch who has a creepy cave filled with creepy and slimy things like in the movie.
Now, unlike the movie the witch cuts out the little mermaid's tongue instead of stealing her voice. So it is a compromise that Disney made and it was a good idea too. However, she takes a potion and passes out from the pain and then finds the Prince and then she tags along and follows him everywhere he goes. The Mermaid then learns that the prince is meant to marry someone from a close by kingdom. The mermaid then realizes that she would die soon if he does not marry her.
Her sisters rise up from the sea and they have cut off their hair to get a special blade for her to use on the prince and once she stabs the love of her life she can turn back into a mermaid by getting a tail from the blood of the freshly dead prince. Disney decided not to put this in because it is a dark side of the little mermaid that should be ignored.
She then plans to do it. She sneaks into the prince's chamber where he and his new bride are fast asleep and she realizes she cannot kill him. She tosses the knife aside and she expects to turn to sea foam, but instead, she is turned into a spirit of the air.
The Disney ending is much more fabulous because Disney made Ursula turn into the bride to be and then that way Ariel would get her happy ending and children all over the world would be happy. Ursula dies and Ariel gets the prince! Just as it was supposed to be.